The present invention relates to a fuel injection pump for a diesel engine, and more particularly relates to an electromagnetic type of diesel fuel injection pump in which fuel spilling is controlled electronically by an electromagnetic valve rather than mechanically by the use of a spill ring or the like.
There is known a type of fuel injection pump for a diesel internal combustion engine which includes a plunger which reciprocates to and fro in a bore defined in a housing, a high pressure chamber being defined between one end of the plunger and the end of the bore. During the suction stroke of the plunger as this high pressure chamber expands in size, diesel fuel is sucked into this high pressure chamber from a quantity of diesel fuel contained in a relatively low pressure chamber, through a fuel supply passage; and during the compression stroke of the plunger as the high pressure chamber subsequently contracts in size, this diesel fuel in the high pressure chamber is squeezed and is brought to a high pressure and is ejected through an injection passage therefor to a fuel injector of the diesel internal combustion engine. Sometimes, in the case that the diesel fuel injection pump is so called distribution type pump, the plunger is rotated as it reciprocates by an input shaft which is rotationally coupled to it although not axially coupled to it, and by a per se well known construction the spirt of highly compressed diesel fuel is directed to the appropriate one of the plurality of cylinders of the internal combustion engine. It is known for the reciprocation of the plunger to be performed by having a cam plate fixedly mounted on the end of said plunger remote from the high pressure chamber and rotating with it. The cam plate is formed with at least one cam protuberance, and a mounting member is mounted to the pump housing so as to oppose the cam plate with at least one roller mounted to this mounting member so as to contact the cam plate. The combination of the cam plate and the plunger is biased towards the roller so as to be pressed against it and so as to cause said plunger to be reciprocated as it rotates by the cam action of the cam protuberance on the roller.
Now, such a fuel injection pump injects an amount of diesel fuel in each pump stroke which is regulated by a fuel injection amount control means which selectively vents the high pressure chamber. This control means ceases to vent the high pressure chamber when it is appropriate to start the fuel injection spirt, during the compression stroke of the plunger, and at this instant the almost incompressible diesel fuel in the high pressure chamber starts to be squeezed and injected, as explained above. When it is appropriate to terminate the fuel injection spirt, then the control means starts again to vent the high pressure chamber, and at this instant the diesel fuel in the high pressure chamber ceases to be squeezed and therefore the injection is immediately stopped. It has been conventional for this high pressure chamber selective venting means to be a spill ring, which is mechanically positioned according to the position of the accelerator pedal which is controlling the engine, and whose position controls the timing instant of the end of the non-vented time period of the high pressure chamber. However, nowadays electronically controlled fuel injection pumps are coming into use, in which the selective venting of the high pressure chamber is performed electronically by an electromagnetic valve which is controlled by an electronic control system such as one incorporating a microcomputer.
In such an electronic fuel injection pump, the electronic control system, for each spirt of fuel injection, calculates how much fuel is to be injected in this spirt, and then at an appropriate time point for the start of fuel injection closes said electromagnetic valve, so as to terminate fuel spilling from the high pressure chamber and so as thereby to start fuel injection. After the electronic control system has calculated that the proper amount of fuel has been injected by the movement of the plunger in the direction to reduce the size of the high pressure chamber, then said control system opens said electromagnetic valve for fuel spilling again, thus immediately terminating fuel injection.
Conventionally, in order for the electronic control system to receive information as to the position of the crankshaft of the diesel engine, in other words as to what point in its cycle the diesel engine has reached, so that the electronic control system may start the fuel injection at the appropriate timing point, there has been provided a signal rotor fixedly mounted on the input shaft of the fuel injection pump, with a signal generator being mounted so as to oppose an outer peripheral portion of the signal rotor, and with a plurality of means, the same in number as the number of cylinders of said diesel engine, being equidistantly spaced around an outer peripheral portion of the signal rotor, for causing the signal generator to output reference position signals when they pass it as said signal rotor rotates. For example, the reference position signal producers may simply be teeth on the rotor, of the same number as the number of cylinders of the diesel engine and equidistantly spaced around the rotor; or alternatively they may be notches in the outer circumferential surface of the rotor. In any case, as each of the reference position signal producers passes the signal generator, it causes the signal generator to emit a characteristic reference position signal, which is transmitted to the electronic control system and informs it that the crankshaft of the engine has reached a particular rotational position, typically one shortly before the start of fuel injection to one of the cylinders is required. From this instant, the electronic control system counts a certain proper time interval until the proper instant for the start of fuel injection, and then at this proper instant closes the electromagnetic valve for fuel spilling. Meanwhile, the electronic control system also calculates how long a time period corresponds to the proper amount of fuel to be injected into the cylinder in this fuel injection spirt, based upon the current revolution speed of the diesel engine, and then counts this time period until the proper instant for the end of fuel injection. At this proper instant, then, the control system opens the electromagnetic valve for fuel spilling, and thus terminates fuel injection.
However, a problem has occurred with this construction, as follows. Since the desired proper amount of fuel to be injected into the cylinder in this fuel injection spirt, which is calculated by the control system based upon certain engine operational parameters, is expressed as a time period which is calculated as based upon the revolution speed of the diesel engine, or rather on a revolution speed value which was determined at some time in the near past, thus when the engine rotational speed is quickly changing, as during acceleration or deceleration, it becomes impossible to inject the correct amount of diesel fuel, and the proper operation of the engine becomes very difficult. This problem can deteriorate the fuel economy of the diesel engine, and also and particularly can deteriorate the quality of the exhaust emissions of the engine.